1. Field of the Invention
The invention relates to a method for the production of light-weight molded parts made of fiber-reinforced polypropylene, wherein a pre-cut part A of a flat semifinished product made of fiber-reinforced polypropylene with a content of reinforcing fibers of 35 to 80 wt-% and a content of uniformly distributed air pores of 20 to 80 vol-% is placed into a two-part mold, heat pressed and thereby molded.
2. Description of the Related Art
Thermoplastically moldable panels of semifinished product made of glass fiber-reinforced polypropylene, so called GMT panels, have already been used for a long time for the production of moldings, in particular for automotive parts. Such “plastic panels”, which generally have a thickness of 0.5 to 3 mm, stand out for their high strength and toughness. Moreover, GMT is capable of flowing, so that it completely fills up the inner contours of the tool during heat pressing of the semifinished product into finished parts. GMT semifinished product is manufactured on a large scale industrially by combining continuous glass mats and polypropylene melt sheets on a double band press. This procedure consumes a large amount of energy because the viscous melt must be pressed into the mat at pressures far above 1 bar. It is thus hardly possible, in practice, to achieve a fiber content greater than 45 wt %.
In order to overcome these drawbacks, semifinished product panels have recently been developed that are also made of glass fiber-reinforced polypropylene but contain air pores with a uniform distribution, so that the finished parts thus produced are lighter and have better noise absorption than finished parts made of GMT semifinished product. Moreover, the production of such air semifinished products containing air pores requires considerably smaller amounts of energy. In practice, they are manufactured by two methods:
In a dry process, e.g. according to WO 2006/105682, polypropylene fibers and glass fibers are blended together, the resulting blended nonwoven is needled, heated to temperatures above the softening point of the polypropylene and compressed on a double band press at pressures below 1 bar.
In a wet process, chopped glass fiber bundles and polypropylene particles are dispersed in an aqueous solution of an emulsifier, whereupon the dispersion is pressed out, dried and heat pressed. The panel thus obtained is thermally expanded to a porous semifinished product.
Such semifinished products containing air pores are generally not capable of flowing, so that a pressforming process has to be used for the production of finished parts. For the production of components having dimensions differing from the form of a rectangle by means of pressing in two-part forming tools it is necessary to either punch out appropriate blanks from the semifinished panels or to do some machining of the parts after pressing, which can also result in substantial amounts of punching waste. In this process, a second, costly operation step is necessary. Moreover, functional parts, e.g. fixation parts that cannot be made with the pressforming process, have to be attached by means of an additional welding step.